1. Field of the Invention
The present invention relates to filter elements used to remove particulate matter from fluid streams. Specifically, the present invention is a high-efficiency, self-supporting filter element made from fibers.
2. Description of the Prior Art
One of the oldest and most reliable methods for separating dispersed particles from a dispersing fluid is the use of porous media in the form of a textile. A typical application is the filtering of fly ash from a gas stream from the burning of coal for power generation before the stream exits from the stack. New regulations for particulate emissions have highlighted deficiencies in conventional fabrics that can be overcome by innovative textile fiber and fabric developments.
The specification for a good filter medium is based on particle retention at an acceptable efficiency and an acceptable life in the filtering environment. The most important decision behind the success of a filtration operation is often the selection of the filter medium. For gas filtration, the medium selection involves the optimization of resistance to flow, resistance to chemical attack, ability to discharge the cake (aggregated particles on the upstream side of the filter) easily and cleanly, resistance to mechanical wear and minimum cost.
Nonwoven fabrics made from fibers are able to meet these criteria for many industrial applications. Flow resistance can be minimized through the right combination of air permeability, pore size and pore size distribution values in the fabric. Textile fibers are available in a wide variety of materials for most filtration environments. To facilitate the release of particles, the surface of a textile may be modified to remove fiber ends that otherwise hold onto particles.
Industrial gas cleaning applications are increasingly using pulse cleaned filters of needlefelt nonwoven fabrics formed into tubes and supported on wire frames. Pulsing is used to periodically clean the filter elements to restore their air permeability to some optimum value. The cleaning itself can contribute to premature fabric failure, often as a consequence of the means used to mount the bag-shaped filter elements. Such means include wire frames, rings and other fabric support hardware, all usually made of metal. These rigid structures over which the fabric filter elements are repeatedly flexed eventually cause the filter elements to abrade and tear, producing holes that necessitate replacement of the filter element. Filter cages in poor condition can also lead to increased abrasion and failure of the filter fabric. There have been many novel devices designed to reduce flexing and abrasion ranging from pulse diffusers to mesh fabrics that give additional support to the filter fabric. Pulsing and pulse pressures have been studied to improve cleaning and fabric wear.
Filtration efficiency is increasingly becoming the most important factor in the selection of a filter medium. The requirement for fine particle filtration is necessary to comply with environmental regulations. Fine particles with aerodynamic diameters of less than 10 micrometers are difficult to collect with conventional needled filter fabrics. Fine particulate material in a gas stream can lead to clogging of needlefelt fabric structures that can increase pressure drop to unacceptably high levels. High pressure drops necessitate more frequent cleaning, and, as a consequence, lead to increased mechanical wear on the fabric. The efficiency may be improved by laminating or coating the needled fabric surface with a fibrous or polymeric membrane. This is an extra and often costly manufacturing step.
Accordingly, the provision of a rigidized fiber filter element that is self-supporting and does not require supporting hardware would represent a significant advance to fabric filter technology. A rigidified fiber filter element would have the added benefit of being easily replaceable by filter elements of a different size. The length, cross-sectional area and shape could be changed to alter the filter area or performance characteristics as demands on the filter are revised. Another significant advantage would be that the efficiency of the rigidized fiber filter element would be greater than that of a conventional fabric due to the density of the fiber assembly in the fibrous structure. The rigidified fiber filter element would achieve a reduction in mechanical wear and an increase in efficiency, while retaining the advantages inherent in fiber and textile technology. Precursor fabrics could be made by any textile fiber processing method.
Filter elements constructed of a single material only may be disposed of more easily. Lack of a supporting frame would simplify the breaking apart and compression of the element for disposal. An element consisting of a single material may be suitable for recycling or incineration as a means of disposal.